Part One: Irradiance Losses
Obstructed Panel Losses: Soiling & Snow
A PV system’s performance is directly tied to how much sunlight reaches the panels.
Obstructed Panel Losses
Soiling Losses
Suggested Soiling Loss Values:
- 5% for regions with long dry seasons
- Add 1-2% if the region experiences frequent dust deposits
- Add 1% if near major traffic areas
- Remove 0.5% if the system will be cleaned in the summer
- 2% for regions with year-round rain
- NREL’s Performance Parameters Paper suggests a 5% typical soiling loss
☀️Soiling losses occur when sunlight is blocked by dirt and debris that accumulates on solar panels over time.
Soiling differs drastically based on your region — everything from the amount of wind, exposed dirt, and local traffic, to the frequency and strength of rain, or even the number of birds, can affect production.
Note: Commercial installers may want to consult with ground monitoring companies to create an on-site soiling estimate since O&M costs are partially dictated by the frequency of cleaning required.
📚 Further reading:
- PV Soiling Rate Variation over Long Periods – Reports a soiling loss of about 0.5%/day in a desert environment.
- Time Series Analysis of Photovoltaic Soiling Station Data – Version 1.0, August 2017 – Reviews soiling at various U.S. locations, finding that these values range 0.03 to 0.15%/day.
Obstructed Panel Losses
Cleaning to Prevent Soiling Losses
Dust accumulates over time, ranging from 0.01%/day up to 0.5%/day in extremely dusty areas. This means that panels should be washed periodically to prevent excessive buildup.
An NREL model found that having one annual cleaning on a system with 1.9% soiling loss would decrease the loss to around 1.5%. Two cleanings per year could drop the average loss to 1.3%, and three cleanings per year would reduce it further to a 1.2% average annual loss.
📚 Further reading: Impact & Consequences of Soiling and Cleaning of PV Module – Finds soiling losses are highly dependent on the local environment and explores the impact of a variety of cleaning methods.
Obstructed Panel Losses
A Note on Birds
Bird droppings are typically counted under soiling losses since they represent a temporary environmental blockage on the panel, but they do have some distinct behavior. Unlike dirt, bird droppings substantially block one or two cells, and may not wash away with rain. Due to how cells are connected within a panel, the impacted cells could knock out the cell string or the entire module if the system doesn’t have bypass diodes. The best advice is to manually clean off the panel quickly.
Obstructed Panel Losses
Snow Cover Losses
Suggested Values:
- A NREL study calculated losses ranging from 10-30% for fixed-tilt systems.
- Aurora suggests using monthly snow loss factors rather than annual ones.
Determining the appropriate loss factor for snow on solar panels is tricky. Not only does the amount of snow vary widely depending on location, the design of the solar installation itself — particularly the tilt of the panels — can play an important role as well. In order to provide an accessible way to determine a representative snow loss, the Aurora team ran NREL’s model for a range of locations in the U.S. and across a range of tilts. From there, the team compiled the appropriate percentage loss factors for each month according to NREL’s research and created this helpful map.
Tips for Reducing Soiling Losses
- In drier climates, use spare water to gently clean panels between rainfall.
- In rainier climates, the rain will handle most dirt.
- Bird droppings have to be cleaned manually and should be addressed quickly.
